Field effect transistors (FETs) are widely used in the electronics industry for switching, amplification, filtering and other tasks related to both analog and digital electrical signals. Most common among these are metal oxide semiconductor field effect transistors (MOSFET or MOS), in which a gate structure is energized to create an electric field in an underlying channel region of a semiconductor body, by which electrons are allowed to travel through the channel between a source region and a drain region of the semiconductor body. Complementary MOS (CMOS) devices have become widely used in the semiconductor industry, wherein both n-type and p-type (NMOS and PMOS) transistors are used to fabricate logic and circuitry.
Continuing trends in semiconductor device manufacturing include a reduction in electrical device feature size (scaling), as well as improvements in device performance in terms of device switching speed and power consumption. Recent MOS and CMOS transistor scaling efforts have focused on high-k dielectric materials having dielectric constants greater than that of SiO2 (e.g., greater than about 3.9), which can be formed in a thicker layer than scaled SiO2, and yet which produce equivalent field effect performance. Another type of CMOS device that is available is one where the gate electrode includes at least a metal layer beneath a Si-containing, e.g., polysilicon, gate electrode.